1. Field of the Invention The present invention relates to a damping circuit for at least two Series-connected turn-off valves according to the preamble of Patent claim 1.
2. Discussion of Background
A known damping circuit is disclosed in the French journal "Electronique de puissance" ("Power electronics"), 11 Sept. 1985, pages 12 and 13. In the circuit represented there in FIG. 3, two GTO thyristors are connected in series via a choke with mid-point tapping. Connected in parallel to each GTO thyristor with antiparallel free-wheeling diode is a series circuit of a turn-off load-shedding capacitor with a turn-off load-shedding diode. The common junction points of the turn-off load-shedding capacitor and turnoff load-shedding diode of the two series circuits are interconnected, on the one hand, via a primary winding of a transformer, the secondary winding of the transformer being connected to the supply direct voltage via a diode, and, on the other hand, via a further series circuit of three diodes and an ohmic resistance. The capacitor ring-around energy can be recovered with this damping circuit. However, the damping circuit is relatively expensive in view of a number of its components. Electric losses occur both in the ohmic resistance, and also in the choke with midpoint tapping.
Concerning the prior art, reference is made in addition to EP-B1-0,134,508. In the circuit represented there in FIG. 4, two GTO thyristors are directly connected in series in each case to a choke with mid-point tapping, there being no interconnection. Parallel to each GTO thyristor, a turn-off load-shedding diode is connected in series to a turn-off load-shedding capacitor. When each GTO thyristor is switched in, the turn-off load-shedding capacitor is discharged via an auxiliary thyristor, especially a breakover voltage thyristor and two diodes, an energy storage choke and the GTO thyristor. Thereafter, a free-wheeling current flows through the energy storage choke, the GTO thyristor and a free-wheeling diode. When the load thyristor is switched out, a portion of the energy temporarily stored in the energy storage choke is used to recharge the turn-off load-shedding capacitor, and a portion is fed back into a direct-voltage source. In this way, an inductive load can be controlled inexpensively via clearly defined switching times. The energy storage choke has an inductance which is approximately 10.sup.4 times as large as the inductance of the circuit composed of GTO thyristor and the series connection, parallel thereto, of turn-off load-shedding capacitor and turn-off load-shedding diode.
Additionally, reference is made to DE-B2-2,641,183. In the low-loss damping circuit given there of two electrical or electronic one-way switches connected in series e.g. two npn transistors, there is provided parallel to each one-way switch, in accordance with a variant, a series circuit of a load-shedding capacitor and a load-shedding diode, the load-shedding diode being of the same polarity as the one-way switch. Parallel to each load-shedding diode, a discharge coil is connected in series to a discharge diode, which has the opposite polarity to that of the load-shedding diode. This damping circuit does not enable recovery of the ring-around energy; it requires 2 discharge diodes and 2 discharge coils. When a one-way switch is turned off, only the respective associated load-shedding capacitor operates.